JOURNAL OF MATERIALS SCIENCE34 (1999 )1533– 1541

Configurations of ferroelectric domains

in bismuth- and Zinc-modified

Pb(Ni1/3Nb2/3)O3-PbTiO3-PbZrO3 ceramics

XINHUA ZHU∗, JIANMIN ZHU, SHUNHUA ZHOU, QI LINational Laboratory of Solid State of Microstructures, Department of Physics, NanjingUniversity, Nanjing 210093, People’s Republic of ChinaE-mail: liqi@netra.nju.edu.cn

ZHONGYAN MENGSchool of Materials Science and Engineering, Shanghai University (Jiading Campus),Shanghai 201800, People’s Republic of China

NAIBEN MINGCCAST (World Laboratory), P.O. Box 8730, Beijing 100080, and National Laboratory of SolidState of Microstructures, Department of Physics, Nanjing University, Nanjing 210093,People’s Republic of China

Transmission electron microscopy was used to investigate the domain structures of the(Pb0.985Bi0.01)(Ni1/4Zn1/12Nb2/3)0.2(ZrσTi1−σ )0.8O3 (0.30≤ σ ≤ 0.70) ceramics, which are locatedin the ferroelectric tetragonal and rhombohedral phase regions, and also near themorphotropic phase boundary (MPB). The results show that the lamellar twinning domainsand the δ-fringe contrast are most frequently observed in the compositions located in theferroelectrc tetragonal phase region. In the compositions near the MPB, a banded domainstructure similar to herringbone pattern is observed, which contains many parallel bandsforming 90◦ or 70◦ angles whereas they are inconsistent with one another on both sides ofthe herringbone domain patterns. The morphology of the herringbone domain structureobserved in the bismuth- and zinc-modified PNN-PZ-PT ceramics with composition near theMPB can be described by a space-stacking succession of two crystallographicallyequivalent plates whereas made from different twin-related domains, with the same habitplane parallel to the (011)-type plane. In the compositions located in the rhombohedralphase region, the stripelike domains are observed, and a local random contrastrepresenting short-range-ordered ‘island’-typed polar clusters or nanodomains is alsofound, which is attributed to the existence of the polar microregions with the dispersednanometer-sized short-range-ordered domains in the rhombohedral matrix, because thefree energy of the ensemble of the polar microregion is lowered, and the relativethermodynamic stability is increased with increasing the content ratio of Zr to Ti. Inaddition, the wavy character in the thickness fringe is commonly observed at the fringe ofthin foil, which is due to continuous bending of the thin foil at various equivalent directions.C© 1999 Kluwer Academic Publishers

1. IntroductionOver the past period the applications of piezoelectricmaterials in the modern techniques have been greatlyenlarged. A great number of piezoelectric ceramic ma-terials have been developed in the world, among whichlead zirconate titanate (PZT) with perovskite struc-ture is one of the best known. Some minor additivesare used to modify and improve its electrical proper-ties, to obtain the materials with the desired param-eters [1, 2]. However, when two or more kind addi-

∗ Author to whom all correspondence should be addressed.

tives are added simultaneously to the PZT solid so-lution, the piezoelectric properties are not greatly im-proved. Consequently much attention has been turnedto introducing a new components with perovskitestructure into PZT solid solution system, to developnew piezoelectric materials [3, 4]. Using this factoris of considerable value for the purposeful modifi-cation of materials properties due to the fact that itcan usually broaden the morphotropic region and givea new dimension to compositional changes. In this

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way, the diversity of possible property combinationscan be greatly increased. Among the A(B′B′′)O3-PZTpseudoternary solid solutions, Pb(Ni1/3Nb2/3)O3-PbTiO3-PbZrO3 (PNN-PT-PZ) ceramics is one of theexcellent materials, which can comprehensively satisfythe property requirements for piezoelectric ceramic ac-tuators [5]. The morphotropic phase boundary (MPB)and increase of the piezoelectric activity therein areobserved in the Bi- and Zn-modified PNN-PZT pseu-doternary system [6], which is similar to that of the PZ-PT system. These phenomena are related to the motionof domain boundaires [7, 8]. It is therefore very impor-tant to examine and interpret the ferroelctric domainstructures appropriately. The domain configurations ofthe Bi- and Zn-modified PNN-PZ-PT solid solutions isof particular interest because of its promising applica-tions in the piezoelectric ceramic actuators [9].

Ferroelectric domains are classified intoa- andc-domains, depending on which axis of the unit celllies parallel to the viewing direction. If a viewing di-rection is one of crystallographic axes, there are fourtypes of domain boundaries, which are named as (I) 90◦a-a boundaries, (II) 90◦ a-c boundaries, (III) 180◦ c-cboundaries, and (IV) 180◦ a-a boundaries. However,(I) and (II) are structurally identical boundaries, as are(III) and (IV), the only difference is the direction ofobservation [10].

Each domain has different optical and etching prop-erties along the polarization directions. Thus, the fer-roelectric domain structures can be visualized by po-larizing light microscopy [11], optical contrast [12],scanning electron microscopy [13], and so on. Trans-mission electron microscopy (TEM) provides the mostdirect method to study the domain structures with muchbetter resolution as well as reciprocal lattice informa-tion [14]. Until recently, TEM studies of ferroelectricdomains in oxide ferroelectrics have mainly been con-fined to the barium titanate crystals and ceramics [15],and there have been few published TEM works on theconfigurations of ferroelectric domains for the Bi- andZn-modified PNN-PZ-PT pseudo-ternary system ce-ramics. In this paper, we present our TEM observa-tions on the ferroelectric domain configurations of theBi- and Zn-modified PNN-PZ-PT pseudo-ternary sys-tem with various compositions, which are located inthe ferroelectric tetragonal and rhombohedral phases,and near the MPB, to find out the relationships betweendomain configurations and electrical properties of thepresent ceramics.

2. Experimental procedureSamples of compositions (Pb0.985Bi0.01)(Ni1/4Zn1/12Nb2/3)0.2(ZrσTi1−σ )0.8O3 with 0.30≤ σ ≤ 0.70 wereprepared by the method described in [4]. The speci-mens for TEM observation were prepared from bulkmaterials by mechanical thinning and then followed byion beam (Ar+) milling using a Gatan dual ion mill(model 600) operated at 5 kV with a combined guncurrent of 1 mA. These specimens were coated withcarbon to prevent charging in the microscope. The do-main configurations were observed by a JEOL JEM-

200CX microscope operated at 200 kV, using a doubletilt stage at room temperature.

3. Results and discussionPrevious results have shown that the composition withσ = 0.70 is located in the ferroelectric rhombohedralphase region, composition withσ = 0.50 near the MPBand composition forσ = 0.30 and 0.40 are located inthe ferroelectric tetragonal phase region. The differ-ent configurations of the ferroelectric domains in theBi- and Zn-modified PNN-PZ-PT ceramics with var-ious compositions are described and discussed in thefollowing.

3.1. Composition in the tetragonalphase region

In the tetragonal phase, there are only the 90◦ and 180◦domains. A typical configurations of ferroelectric do-mains observed in the composition withσ = 0.30 isshown in Fig. 1, in which several inclined grain bound-aries are observed, as indicated by 1-1, 2-2, 3-3 and4-4. The grain A and C are subdivided by fine blackand white stripes, which are sets of parallel{110}-type90◦ a-a domain boundaries. Closely packed “stripes”of domains are seen in the grain B, as shown in Fig. 1,which shows the characteristics of theδ-fringe contrast.The lamellar domains with periodicity of about 0.2µmcan be observed in the grain D, as illustrated separatelyin Fig. 2. Within the lamellar domains, the coherentδ-boundaries revealing theδ-fringe contrast are alsoobserved. For composition withσ = 0.40, the domainconfigurations are much more complex than that ofcomposition withσ = 0.30. The 90◦ a-a domains withvarious configurations are shown in Figs 3a–c, whichexhibit (I) periodically regular stripes showing light ordark contrast, (II) parallel bands without constant pe-riodicity, and (III) parallel wedge-shaped domains. Aconventional 90◦ a-cboundary is observed, as shown inFig. 4. It is noticed that this boundary is geometricallyidentical with the conventional 90◦ a-a boundary, ex-cept viewed from a different direction. Fringe contrastis also observed due to that the boundary is inclinedat 45◦ to the plane of the foil. For this type of bound-ary the line of the interaction of the boundary with thefoil surface lies along the [010] direction. Similar TEMobservations have been found in Ca-doped BaTiO3 ce-ramics [10]. In addition, theδ-fringe contrast is alsoobserved within the lamellar 90◦ domains with peri-odicities of about 100, 180 and 200 nm, as shown inFigs 5a–c respectively.

3.2. Composition near the MPBWithin the composition near the MPB between the fer-roelectric rhombohedral (R) and tetragonal (T) phases,there are 14 crystallographic directions in which thedipole can be oriented, and therefore the peak piezo-electric modulus occurs for this composition [6]. It isknown that 180◦ domain walls will appear identical forthe R and T ferroelectric phases due to not needing to

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Figure 1 Bright field image of the typical configurations of ferroelectric domains in the composition withσ = 0.30, in which the inclined grainboundaries are indicated by 1-1, 2-2, 3-3 and 4-4. The grain A and C are subdivided by sets of parallel{110}-type 90◦ domain boundaries. The lamellardomains andδ-fringe contrast are observed in grain B and D respectively.

Figure 2 BF images of the lamellar domains taken from the grain D, the same place as indicated in Fig. 1. Theδ-fringe contrast is also observed inthe grain D.

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Figure 3 BF images of the 90◦ a-a ferroelectric domains with different configurations in the composition withσ = 0.40, (a) periodically regularstripes showing light or dark contrast, (b) parallel bands without constant periodicity, and (c) parallel wedge-shaped domains.

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Figure 4 (a) BF image of the 90◦ a-c boundary in the composition withσ = 0.40, (b) schematic representation of domain boundary.

lie on any one set of crystallographic planes, and thusthe 180◦ domain wall boundaries are usually wavy. Fornon-180◦ domains, there are the 90◦ domains havingwalls which lie on the{110} planes for the T phase,and in the R phase 109◦ and 71◦ domains having wallswhich lie on the{110} and{100} planes respectively.In the morphotropic phase region, the 90◦ domains arepredominant [16, 17].

Fig. 6 is a bright field image of the domain structurein the morphotropic sample with compositionσ = 0.50.It can be seen that the domain walls lie parallel with the{110} planes and quite regular. It is possible that the 71◦domains are present which have walls lying on the{100}planes, however, it is not a common observation and thevast majority have been shown to be the 109◦ domainswhich have walls that lie on the{110} planes. The rea-son is that in the R phase a twin boundary on the{110}planes has a surface energy one-third of that of a twinboundary on the{100} planes. That favors the occur-rence of the{110}-type twin domains [18]. Therefore,the image in the Fig. 6 is a fairly typical observation.Interactions between these types of domain walls canlead to complex herringbone domain texture, which areshown in Figs 7a and b, in which the domain thicknessvaries in range of 40 to 200 nm. Many parallel bands areobserved on the both sides of the herringbone domain

structures, which form 90◦ and 70◦ angles mutually,as shown in Figs 7a and b respectively. These angularvalues can be explained by means of a spatial configu-ration for the domain structure proposed previously forBaTiO3 ceramics [19]. In the herringbone domain pat-tern observed in compositions near the MPB, it is foundthat there is no the one to one correspondence of theparallel bands on both sides of the herringbone domainpattern. This phenomenon is similar to that observed inBaTiO3 ceramics [20]. Considering a plate consistingof thin related variants with the habit plane parallel tothe (011)-type plane and another crystallographicallyequivalent plate with the same orientation of the habitplane, but made of a different twin-related variants, theherringbone domain pattern can be readily obtained byalternatively space-stacking succession of these plates.

3.3. Composition in the rhombohedralphase region

The domain configurations in the composition withσ = 0.70 are shown in Fig. 8, in which the micro-sizedrhombohedral ferroelectric domains with stripelike pat-terns can be observed, and some ‘island’-typed po-lar nanodomains that exhibit a local coarse contrastare also observed, as indicated by an arrow on the

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Figure 5 BF images of the lamellar domains with periodicity of about (a) 100 nm, (b) 180 nm and (c) 200 nm, in which theδ-fringe contrast isobserved.

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Figure 6 A bright field image of the typical domain structure in the morphotropic sample withσ = 0.50.

Figure 7 BF images of the herringbone domain structures in the morphotropic sample withσ = 0.50. The nearly 90◦ and 70◦ angles are formedbetween the parallel bands on both sides of the herringbone domain pattern, which are shown in Figs 7a and b respectively.

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Figure 8 BF images of the domain configurations in composition withσ = 0.70 in the rhombohedral region, in which micro-sized domains withstripelike patterns and some ‘island’-typed polar nanodomains are observed.

Figure 9 (a)–(d) The observed wavy character in the thickness fringe at the fringe of the thin foil.

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micrograph. The local random contrast representing theshort range ordered polar clusters or nanodomains isobserved due to the existence of dispersed nanometer-sized domains in the rhombohedral matrix, because therelative thermodynamic stability of the polar micro-region is increased with increasing the content of Zr toTi [21]. In addition, the wavy character in the thicknessfringe is also observed at the fringe of thin foil, as shownin Fig. 9a. This phenomenon is frequently observed, asdemonstrated in Figs 9b–d. The waviness in the thick-ness fringe is attributed to the continuous bending ofthe thin foil at various equivalent directions.

4. ConclusionsThe configurations of the ferroelectric domains inthe Bi- and Zn-modified Pb(Ni1/3Nb2/3)O3-PbTiO3-PbZrO3 (PNN-PT-PZ) ceramics with compositions lo-cated in the ferroelectric tetragonal and rhombohedralphases, and near the morphotropic phase boundary havebeen investigated by transmission electron microscopy.The lamellar 90◦ domains andδ-fringe contrast are fre-quently observed in the compositions with the ferro-electrc tetragonal phase. The micro-sized rhombohe-dral ferroelectric domains with stripelike patterns and alocal random contrast representing short-range-orderedpolar cluster or nanodomains are observed in the com-position with ferroelectric rhombohedral phase. At themorphotropic phase boundary region, a complex her-ringbone domain structure is observed, in which manyparallel bands form 90◦ or 70◦ angles mutually whereasthey are inconsistent with one another on both sides ofthe herringbone domain patterns. The crystallographyand morphology of the herringbone domain structureobserved in the composition near the MPB can be ex-plained by a stacking succession of two crystallograph-ically equivalent plates but made from different twin-related domains, with the same orientation of the habitplane parallel to the (011)-type plane. The observedwaviness in the thickness fringe is due to the continu-ous bending of the thin foil at equivalent directions.

AcknowledgementsThe authors would like to acknowledge the use of JEOLJEM-200CX TEM at the Analytical Center of NanjingUniversity. This work is supported by the China Post-doctoral Science Foundation and the Opening Projectof National Laboratory of Solid State Microstructures(NLSSMS), Nanjing University.

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Received 16 October 1997and accepted 9 November 1998

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